Curr Biol. 2019 Oct 7;29(19):3193-3199.e4. doi:10.1016/j.cub.2019.07.092

Henriet S, Colom Sanmartí B, Sumic S, Chourrout D.

Evolution of the U2 Spliceosome for Processing Numerous and Highly Diverse Non-canonical Introns in the Chordate Fritillaria borealis.

In Fritillaria borealis, most ancestral GT/AG canonical introns were lost. New AG/AR introns with non-canonical splice sites were created by the insertion of MITEs (miniature inverted-repeat transposable elements) after TA sites. Splicing of non-canonical introns can be inhibited with Pladienolide B. Non-canonical introns of F. bor and O. dioica genes can not be spliced in HEK293T cells, but canonical introns can. In ~90% of lariats from F. bor or O. dio, the branchpoint is an A. Distance to 3′ splice site is similar in both species. A single U2 spliceosome was found in F. bor.